Deep diving is, put simply, getting to know someone.
You can definitely get to know someone at any age.

Sure, high schoolers don't have much experience in life, but you don't need to have much experience in life to have dreams, aspirations, passions, and hobbies.
However, I feel like a lot of people (especially younger people) haven't explored the "why" behind things though. Why do you do what you do? Why do you feel this way? Why are you like this? Questions like these are not always easy to answer and get you pretty deep in conversation. I'm older now but when I was in high school, I NEVER had deep conversations. It was always just surface level conversation. You must keep in mind that a lot of high schoolers are probably used to surface level conversation. If you start asking really deep questions about them, they'll start feeling like they are lost at sea with no map. It’s uncharted territory for them because they are not used to being in a deeper state of mind.
So...just keep in mind that you can definitely deep dive in high school but you might hit some snags if you start trying to figure out the why behind people. If you're talented at deep diving, you can lead the conversation and help them explore themselves. This is a really good feeling for people. They'll discover new things about themselves while in conversation with you!